The present invention relates to an optical recording disc.
The optical recording disc is categorized into a write-once optical disc which uses an organic dye for the recording material, such as CD-R and DVD-R, and a rewritable type optical disc which uses a phase-change recording material for the recording material, such as CD-RW and DVD-RW. In each of those optical recording discs, a recording layer is formed on a disc substrate in which a tracking groove (guide groove) is spirally formed. Record marks are formed in the groove to thereby effect the groove recording.
In the optical recording disc, it is necessary to previously record pre-format information, such as synchronization signal and address information, in a surface of the disc substrate. The pre-format information may be recorded by wobbling the groove. The pre-pits having the pre-format information may be formed instead of the groove wobbling. In a case where the pre-format information is recorded by using only the groove wobbling, if the recording track pitch is narrowed in order to increase the recording capacity, it is impossible to obtain a sufficiently high modulation degree. Further, signal leakage from the adjacent groove occurs to possibly deteriorate the CNR (carrier to noise ratio). To cope with this, in the DVD-R and DVD-RW in which information is recorded at high density, the groove is wobbled and further a land pre-pit format is employed in which pre-pits are formed in areas (called lands) each between adjacent grooves. In the land pre-pit format, a recording or reproducing laser beam is tracked by the groove and converged onto the groove. The pre-pits are detected by reading a difference signal derived from a light receiving element, which is divided into two segments in the radial direction of the optical disc.
A general method of manufacturing the disc substrate will be described.
A disc substrate is manufactured in a manner that resin is injection molded by using a stamper having a matrix pattern for the pre-pits and grooves. Usually, the stamper is made of Ni (nickel). To manufacture the stamper, a master disc is formed, and then a master stamper, a mother stamper, a child stamper and the like are formed from the master disc.
The master disc is generally manufactured in the following steps. To start with, a resist layer made of resist material, such as a photo-resist, is formed on a surface of a rigid substrate made of glass. The resist layer is exposed by an exposure beam, such as a laser beam, to thereby form a latent image pattern on the resist layer, and then the latent image pattern is developed. In this way, the resist layer is patterned to complete the master disc.
To form a stamper by using the master disc, a Ni thin film is formed by sputtering or electroless plating to give the surface of the resist layer of the master disc an electrical conductivity. Next, electrocating process is carried out using the Ni thin film as a substrate to thereby form a Ni electroformed film. A laminated layer consisting of the Ni thin film and the Ni electroformed film is peeled from the resist layer, and is used as a master stamper. The mother stamper is manufactured in a manner that a Ni electroformed film is formed on the surface of the master stamper, and the Ni electroformed film is peeled therefrom. In this case, preparation is made in advance such that the surface of the master stamper is oxidized so as to make it easy to peel off the Ni electroformed film. By similar process, the child stamper is manufactured by using the mother stamper.
In the manufacturing process of the master disc, when a latent image pattern corresponding to a spiral groove is formed, the beam is spirally moved for scanning. To form pre-pits between the grooves, two beams are used for forming a latent image pattern. One of the beams is continuously irradiated to form a groove pattern, while the other beam is intermittently irradiated to form a pre-pit pattern. The method for forming the latent image pattern by using two beams will be referred to as a xe2x80x9ctwo-beam methodxe2x80x9d in the specification. A partial plan view of a disc substrate thus manufactured by the two-beam method is shown in FIG. 3A, and a cross sectional view of the same taken on line Bxe2x80x94B in FIG. 3A is shown in FIG. 3B. In FIG. 3A, grooves 4D, 4E and 4F and lands 3D, 3E and 3F are alternately arranged on a disc substrate 2. A pre-pit 5B is formed in the land 3E. The pre-pit 5B is continuous to two grooves 4D and 4E adjacent to the pre-pit. Those grooves and pre-pits are formed in a ladder shape, as a whole.
Japanese patent publication Nos. 2000-200446 and 2000-353321 point out the following problem. In an optical disc in which the grooves and the pre-pits are formed in a ladder shape, when record marks or information pits are formed at positions adjacent to the land pits, the record marks and the information pits expand onto the land pre-pits. As a result, such a problem arises that it is impossible to accurately reproduce the land pre-pit signals. In Japanese patent publication No. 2000-200446, the groove depth and the land pre-pit depth are different from each other in order to suppress the expansion of the record mark onto the land pre-pit. In Japanese patent publication No. 2000-353321, the width of the groove is narrowed at a position adjacent to the land pre-pit. With this, even in the case of the information pit formed extending onto the land pre-pit, a ratio of the pit area to the pit length is equal to that in the case of the normal information pit.
Japanese patent publication No. 2000-187887 proposes such a technique that in order to accurately reproduce preformat information of the land pre-pits, the land pre-pit is formed while leaving one part of the inner or outer peripheral side of the land.
Japanese patent publication Nos. 2000-187887 and 2000-353321 propose such a technique that to form the grooves and the land pre-pits having shapes as described in those publications, when the master disc is formed, a part of the groove is curved toward the groove adjacent to the former by deflecting or biasing one exposure beam (see FIG. 6 in Japanese patent publication No. 2000-187887 and FIG. 12 in Japanese patent publication No. 2000-353321). In this technique, the curved part of the groove serves as a land pre-pit. On the technique, the Japanese patent publication No. 2000-187887 points out such an advantage that since no laser beam for the pre-pit formation is required, the optical system of the master disc exposure device is simple.
In the optical recording disc of the type in which a part of the groove is curved and used for the land pre-pit, the shape of the land pre-pit is greatly different from that in the conventional optical disc, which is manufactured from the master disc by the two-beam method. Therefore, it is estimated that the conditions that the land pre-pit should satisfy in order to obtain good reproduction characteristics are greatly different from those for the conventional optical disc. However, little consideration is given to those conditions in both the publications referred to above.
Accordingly, an object of the present invention is to reproduce both the pre-pit reproducing signal and the record mark reproducing signal in an excellent level in an optical recording disc of the type in which the groove recording is carried out, and one of two adjacent grooves is curved in part toward the other groove and the curved part serves as a land pre-pit.
1) To achieve the above object, there is provided an optical recording disc having a disc substrate and a recording layer, in which a spiral or concentric groove is formed in a recording-layer forming surface of said disc substrate, and the groove recording is performed, wherein a groove is curved in part toward another groove adjacent to said curved groove to thereby form a land pre-pit, and wherein the following relations hold among DG, DP and r:
0.267xe2x89xa6(DP/DG)2xc2x7rxe2x89xa60.701;
and
3.185xe2x89xa6(DP/DG)/rxe2x89xa68.553,
where
DG: depth of the groove having the land pre-pit,
DP: depth of the land pre-pit,
r (xcexcm): distance between the center of the land pre-pit and the center line of the groove having the land pre-pit curved therefrom.
2) In the optical recording disc thus constructed, said land pre-pit and said groove may be shaped like V in cross section.